1. Field of the Invention
This invention relates generally to radiation dosimetry techniques.
2. Related Art
The measurement of neutrons and heavy charged particles (HCP) remains one of the most challenging tasks in radiation dosimetry. Among the most widely investigated and used passive detector technologies are: plastic nuclear track detectors (PNTD) based on CR-39 plastic and its derivatives, pairs of 6LiF and 7LiF thermoluminescent detectors (TLD) in an albedo configuration and superheated bubble detectors (SBD). See Benton et al., “Proton Recoil Neutron Dosimeter for Personnel Monitoring. Health Phys., 40, pp. 801–809 (1981); Piesch et al., “Albedo Dosimetry System for Routine Personnel Monitoring,” Radiat. Prot. Dosim., 23(1/4), pp. 117–120 (1988); d″Errico, “Radiation Dosimetry and Spectrometry with Superheated Emulsions,” Nuclear Instr. Meth. B, 184, pp. 229–254 (2001). TLDs have the strong neutron energy dependence and the difficulty in discriminating between radiations having low- and high linear energy transfer (LET). PNTDs have good neutron/gamma discrimination but require laborious wet-chemistry processing and have low saturation fluence. SBDs are very sensitive to neutrons but bulky and environmentally unstable. All these difficulties have stimulated the search for a new approach.
The measurement of neutrons presents a special problem. Neutron radiation is not a directly ionizing type of radiation and requires transformation or conversion into ionizing radiation (e.g., electrons or heavy charged particles) that can detected by detectors based on ionization of the detector medium. Detection of thermal neutrons requires nuclear reaction and conversion with isotopes such as 6Li or 10B having large thermal neutron capture cross-section. Fast neutron detection is most efficient using plastic converters containing high concentration of hydrogen. Having the same mass as a neutron, a proton can accept most of the kinetic energy of a neutron in head-on collision. The high energy recoil protons generated by these collisions are then able to cause ionization in the detector medium. The amplified signal from the active or passive detector is then processed and provides dosimetric or spectroscopic information about the radiation field.
Dosimeters and methods for detecting neutrons and heavy charged particles based on optically stimulated luminescence, thermoluminescence, and other luminescent and fluorescent techniques have been disclosed in the art. For example, an optically stimulated luminescent, fast neutron sensor and dosimeter is disclosed in U.S. Pat. No. 6,140,651 (Justus et al.), issued Oct. 31, 2000. The disclosed fast neutron sensor and dosimeter comprises a proton radiator with a doped glass, such as Nd-doped glass containing ZnS:Cu and can be read by either laser heating or infrared stimulation of the glass or by direct scintillation.
Thermoluminescent dosimeters and methods for reading thermoluminescent radiation are also disclosed in U.S. Pat. No. 4,638,163 (Braunlich et al.), issued Jan. 20, 1987; U.S. Pat. No. 4,825,084 (Braunlich et al.), issued Apr. 25, 1989; U.S. Pat. No. 4,839,518 (Braunlich et al.), issued Jun. 13, 1989; and U.S. Pat. No. 5,015,855 (Braunlich et al.), issued May 14, 1991. The disclosed dosimeters and methods measure ionizing radiation, particularly heavy charged particles emitted from radioactive materials and other heavy charged particle radiation sources by laser heating and thermoluminescence of phosphors, using, for example, a thin layer of thermoluminescent phosphor material and an inorganic binder heat bonded to a substrate, as described in U.S. Pat. No. 4,825,084 (Braunlich et al.).
A fluorescent glass dosimeter for reading a radiation dose is also disclosed in U.S. Pat. No. 5,057,693 (Burgkhardt et al.), issued Oct. 15, 1991. The disclosed dosimeter reads a radiation dose from a fluorescent glass element, where the radiation dose is determined from the intensity of the fluorescence emitted from the glass element's detecting face. A fluorescence diaphragm arrangement is provided so as to overlay the glass element detecting face and is movable thereon for changing the fluorescence detecting areas and a fluorescence intensity reading device is provided for determining fluorescence intensity distribution and the glass element detecting face.
A neutron dosimetry method, dosimeter and system are also disclosed in U.S. Pat. No. 5,319,210 (Moscovitch), issued Jun. 7, 1994 and U.S. Pat. No. 5,498,876 (Moscovitch), issued Mar. 12, 1996. The method, dosimeter and system disclosed stores information in a three dimensional fluorescent optical memory element that is altered by exposing the optical memory element to neutron radiation and dosimetric information is subsequently retrieved and analyzed by readout of the altered data with the laser system. One described optical memory element is a three dimensional optical random access memory (ORAM) comprising a volume of a transparent polymer doped with a light sensitive chemical such as spirobenzopyran, which is also described in Moscovitch et al., “Radiation Dosimetry Using Three-Dimensional Optical Random Access Memories,” Nucl. Inst. Meth. Phys. Res. Vol. 184 (2001), pp. 207–18.
Unfortunately, the lower efficiency of detection and poorer discrimination between absorbed doses induced by heavy charged particles versus gamma radiation, remain as obstacles to the dosimetry of neutrons by prior dosimeters and methods. Prior dosimeters and methods can have limited spatial resolution, are sometimes not provided with imaging and/or spectroscopic capabilities or systems, may be able only to detect strongly penetrating photon and beta radiation, and may not be able to detect and image individual track of heavy charged particles. In particular, in integrating thermoluminescent and optically stimulated luminescent detectors, the small amounts of intense fluorescence produced within the particle track can be masked by the luminescence occurring from the significantly larger crystal volume as a result of photon and electron interactions. Thermoluminescent detectors and methods are also not able to detect every heavy charged particle incident and can have very low detection efficiencies. In addition, thermoluminescent methods are very slow, requiring tens of milliseconds per data point. Dosimeters and methods based on organic memory materials such a three-dimensional ORAM require pre-recorded optical data stored in the memory medium, require several spatially distributed bits to be affected by radiation and may not posses the sensitivity and spatial resolution required for imaging individual tracks of heavy charged particles.